def area(r):
    s = r * r * 3.1415926
    return s


def main():
    while 1:
        r = float(input("请输入圆的半径，计算圆的面积：\n"))
        if r <= 0:
            print("圆的半径不能为负数,请重新输入\n")
        else:
            area_s = area(r)
            print("圆的面积为:", format(area_s, '5.2f'))
            break


main()
